1. Field of the Invention
The present invention relates to a high-frequency wideband amplifier.
2. Description of the Prior Art
It is known that a power amplifier working as a class AB, B or C amplifier has a non-linear behavior.
This non-linear behavior takes the form notably of a variation of the current consumed by the amplifier as a function of the level of the amplified signal. The supply sources of the collector and of the base of the bipolar transistor or transistors with which such amplifiers are generally fitted out are considered to have zero impedance with respect to the transistors. If not, the resultant variations in voltage lead to an amplitude modulation of the amplified signal that gets added to the original modulation of the signal. These supply sources are generally not directly applied to the base and the collector of the transistor or transistors of the power stage, but by means of decoupling coils between the DC current and the high frequencies. These coils are also often called chokes.
Conventionally, a first type of high-frequency power amplifier is formed essentially by a power stage, comprising one or more parallel-mounted transistors that are connected respectively to the input and the output of the amplifier circuit by means of an impedance-matching circuit, or impedance transformer. The input and output impedance transformers respectively match the impedances presented by the base and the collector of the transistor with the impedance of the high frequency circuit, generally the characteristic impedance of the circuit Z.sub.c =50 Ohms. The supply voltages of the transistor are applied respectively to its collector and base by means of a coil or choke. The impedance presented by the choke should be, firstly, sufficiently great with respect to the input and output impedances of the transistor in the range of the working frequencies of the amplifier, so that its effect is negligible on the high-frequency operation of the amplifier and, secondly, as low as possible in the videofrequency baseband of the signal to be transmitted, which is of the order of 0 to 10 MHz for a television signal. Consequently, the voltage developed between the ends of each coil is reduced to the maximum and consequently makes it possible to prevent an additional modulation on the bias voltages that could prompt a distortion of the signal to be amplified. One of the drawbacks of this assembly is the difficulty of arriving at a satisfactory compromise to determine the value of this impedance.
A second type of assembly, which is akin to the foregoing assembly and has a symmetrical structure, uses two transistors or a dual transistor and makes possible to facilitate the impedance matching between the external circuit with 50 .OMEGA. characteristic impedance and the impedances presented by the amplifier stage.
The DC bias voltages of the transistors are applied respectively to the base and the collector of each transistor by means of a choke and are therefore still subjected to the imperatives of the foregoing assembly.
In a third known type of amplifier assembly, input/output balancing transformers are used to convey the bias voltages respectively to the base and the collector of each transistor of the dual transistor. This assembly makes it possible to obtain a "cold" point for the high-frequency signal, namely a point that is decoupled with respect to the high frequencies and that has the respective voltage source at the base and collector of each transistor applied to it. Thus, this third assembly makes it possible to avoid the need for the foregoing compromise.
In this assembly, the dual transistor is considered to work as a differential amplifier. A continuous link is also provided between the point of application of the base voltage and the base of each transistor, it being assumed that the input impedance transformer has a continuous link between its input terminals and its output terminals. The collector circuit is deduced from the input circuit described here above by symmetry with respect to the amplifier stage. In this last-named type of amplifier, the supply voltages of the base and of the collector of each transistor are applied to cold points for the high frequencies, thus making it possible to do away with the constraints entailed by chokes in the videofrequency baseband.
However, not all the problems are resolved by this last-named assembly, and new drawbacks appear. Thus, the impedance transformer has a continuous link between its input and output terminals and this may be an irksome constraint, and the impedances presented by the balancing transformer and the impedance transformer in the link between the point of application of the base/collector voltages of the transistor remain high in the baseband of the amplifier.